The field of the invention is tubular connections, which are expanded downhole, and more particularly threads that retain makeup torque despite longitudinal shrinkage resulting from expansion downhole.
Oilfield tubular joints comprise a threaded pin and box which are made up against one or more torque shoulders with a force than can exceed 3000 foot pounds. Some designs further incorporate a resilient seal. One of the problems with such seals is that they fail because of a residual clearance in the seal area between the pin and the box, after makeup. When such joints are used downhole and expanded, longitudinal shrinkage at the pin end results in a pulling away of the pin end from the torque shoulder on the box. This allows very low torque levels to undo the connection with values as low as 200 foot-pounds or less. This longitudinal shrinkage can also eliminate the metal-to-metal seal, which existed after makeup and before expansion. Additionally, an advancing swage can impart a rotational movement, which can undo the threaded connection in the presence of the longitudinal shrinkage previously described.
The present invention seeks to overcome, or at least alleviate, these shortcomings of the prior designs. It provides for a locking feature that retains the makeup torque and that is energized due to the expansion. It reduces or eliminates clearance around resilient seals due to the expansion. It also seeks to enhance the locking feature using spaced seals, which can trap atmospheric pressure between them, such that upon insertion downhole, hydrostatic pressure can push the box into the pin to better engage a locking feature. In a preferred embodiment at least one of the pin or box threads are treated with a material designed to reduce galling. The effect of this coating is to harden the surface to which it is applied. Upon expansion after makeup, this hardened surface crumbles, with the particles locking the thread against backing off. These and other advantages of the present invention can be more readily appreciated by a review of the detailed description of the preferred embodiment, which appears below.
A thread for expandable downhole tubular connections is disclosed. The thread on at least one of the pin and box is coated, preferably with an anti-galling compound. During expansion the coating crumbles and the particles in between the threads keep the joint from becoming undone despite longitudinal shrinkage from the expansion or any tendency of the swage to impart a rotational motion on one part of the joint during the expansion process.